Metalloids are elements that share properties of both metals and nonmetals, making them uniquely versatile in chemistry and industry. Six elements are universally recognized as metalloids: boron, silicon, germanium, arsenic, antimony, and tellurium. Polonium and tennessine sometimes make the list, though their classifications are debated. These elements sit along a zigzag line on the periodic table, forming a border between the metals on the left and the nonmetals on the right.
Where Metalloids Sit on the Periodic Table
The zigzag line (sometimes called the stair-step line) runs diagonally from boron in the upper right down toward polonium in the lower right. Elements touching this line tend to have intermediate characteristics, which is exactly what makes them metalloids. The line is more of a guide than a strict rule, and chemists disagree on a few edge cases. Astatine, for example, is sometimes included, and tennessine (element 117) is predicted to behave like a metalloid based on its position, though it’s too unstable to test in any practical way.
Physical Properties
Metalloids generally have a metallic luster, giving them a shiny, reflective surface that looks like metal. But pick one up and try to bend it, and it behaves like a nonmetal: metalloids are brittle and shatter under stress rather than bending or stretching. This combination of looking like a metal while breaking like a nonmetal is one of their most distinctive traits.
Several metalloids exist in multiple structural forms called allotropes. Silicon is a good example. In one form it appears as a brown powder (amorphous silicon), while in another it forms the shiny, gray crystals used in computer chips (crystalline silicon). These different structures give the same element very different physical behaviors.
Metalloids tend to have relatively high melting points compared to most nonmetals. Silicon melts at 1,414°C, which is far above everyday nonmetals like sulfur (115°C) but still below iron (1,538°C) and many other metals. Their densities and boiling points also fall somewhere between typical metals and nonmetals, reinforcing their “in between” identity.
Electrical Conductivity and Semiconduction
The most commercially important property of metalloids is their behavior as semiconductors. Metals conduct electricity well, nonmetals generally don’t, and metalloids fall in between. More importantly, their conductivity changes with temperature in the opposite direction from metals: as a metalloid gets hotter, it conducts electricity better.
This happens because of how electrons are arranged in the solid crystal. In a semiconductor like silicon or germanium, electrons are normally locked in place. But when heat (or light, or an applied voltage) adds energy, some electrons break free and can move through the material. Each freed electron also leaves behind a gap, or “hole,” which neighboring electrons can hop into. Both the moving electrons and the shifting holes carry electrical current, so conductivity rises with temperature.
The amount of energy needed to free those electrons is measured by a property called the band gap. Silicon has a band gap of about 1.11 electronvolts at room temperature, while germanium’s is smaller at roughly 0.66 electronvolts. The smaller gap means germanium conducts more easily than silicon at the same temperature, though both conduct far less than a true metal like copper. This tunable conductivity is the foundation of modern electronics, from transistors to solar cells.
Chemical Properties
Metalloids form covalent bonds, sharing electrons with other atoms much the way nonmetals do. Their crystal structures are built on these shared-electron networks. Silicon, for instance, bonds to four oxygen atoms in a tetrahedral shape, creating the continuous three-dimensional framework found in quartz and most of Earth’s minerals. Arsenic atoms each bond to three neighbors in layered sheets, while tellurium forms long spiral chains where each atom connects to two others.
Different metalloids exhibit different oxidation states, meaning they can gain or lose varying numbers of electrons depending on what they’re reacting with. Silicon typically shows a 4+ state. Arsenic and antimony commonly appear in 3+ or 5+ states. Tellurium usually shows 2+ or 4+, with a strong resistance to reaching its maximum possible state of 6+. Boron most often appears as 3+, but its electron-poor nature allows it to form unusual bonding arrangements, including fractional oxidation states in compounds called boron hydrides.
Amphoteric Oxides
One of the more interesting chemical behaviors of metalloids is that their oxides are often amphoteric, meaning they can react as either an acid or a base depending on what they encounter. Mix an amphoteric oxide with an acid and it behaves like a base, producing a salt and water. Mix the same oxide with a base and it flips, acting like an acid. This dual personality mirrors the metalloids’ broader theme of straddling two categories. Silicon dioxide, for example, sits right at the transition point on the periodic table where oxides shift from basic (like those of sodium and magnesium) to acidic (like those of sulfur and chlorine).
Key Uses in Industry and Technology
Silicon dominates the semiconductor industry. Crystalline silicon is the primary material in computer chips, transistors, and most solar panels. Its abundance in Earth’s crust and its well-understood electrical properties make it the backbone of modern electronics.
Germanium also finds use in transistors and fiber optics, particularly for infrared lenses and night-vision equipment, because it transmits infrared light effectively. Boron is essential in plant biology and is widely used in heat-resistant glass (like Pyrex), ceramics, and as a neutron absorber in nuclear reactors. Antimony goes into flame retardants and is alloyed with lead to harden batteries and ammunition. Tellurium is used in specialized solar cells and as an additive in steel and copper alloys to improve machinability.
Toxicity and Health Effects
Not all metalloids are equally dangerous, but arsenic and antimony pose serious health risks. Arsenic is classified as a group 1 human carcinogen. It can accumulate in and damage every organ, with the highest concentrations building up in the liver and kidney. Chronic exposure is linked to skin disorders, cardiovascular disease, diabetes, and cancers of the lung, kidney, liver, and prostate. It is also a well-established neurotoxin that impairs cognitive function and memory, with children being especially vulnerable.
Arsenic’s toxicity works through multiple pathways. One form (arsenate) mimics phosphate, a molecule cells need for energy production, and interferes with normal metabolism. Another form (arsenite) binds tightly to proteins and disrupts their shape and function. Epidemiological studies have found health effects at drinking water concentrations as low as 10 parts per billion, which is the current safety limit set by many regulatory agencies.
Antimony exposure, while less studied, can affect the skin, respiratory system, cardiovascular system, and gastrointestinal tract. Both arsenic and antimony cause damage at the cellular level through oxidative stress and disruption of DNA, which is why long-term exposure to either is associated with increased cancer risk.